1. Field of the Invention
An object of the present invention is a device to multiply low-noise charge carriers of a given type (electrons or holes), where the multiplication occurs by an impact ionization process so that a charge carrier of a given type is capable of releasing one or more charge carriers of the same type by impact, the said charge carriers being capable in turn of releasing additional charge carriers of the said type, thus constituting an avalanche phenomenon.
In a device for the avalanche multiplication of charge carriers, comprising a semiconductor, the noise depends on the ionization ratios of the two types of charge carriers (electrons and holes) present in this semiconductor device, and on the mechanism that sets off the avalanche multiplication process, the ionization ratio of a charge carrier being the probability per unit of length that this charge carrier will undergo an ionizing impact. The noise of the avalanche device is low if the charge carriers to be multiplied have an ionization ratio which is far greater than that of the opposite charge carriers. A second source of noise in a device of this type is related to the dispersal of the number of ionizing impacts per initial charge carrier. This "statistical" noise of the device is reduced if the multiplication of the said charge carriers in the semiconductor takes place not uniformly but, on the contrary, at certain precise positions in the material.
To obtain a good signal-noise ratio in a charge carrier avalanche multiplication device, it is therefore necessary to reduce the two types of noise, described in the above two paragraphs, to the minimum.
2. Description of the Prior Art
There is a device of this type in the prior art. It is described by F. Capasso et al in an article published in Applied Physics, Lett. 48 (19), 12 May 1986, page 1294. This device comprises an electrically semi-conductive material consisting of a succession of substantially parallel heterojunctions between semiconductors of different natures (the term heterojunction refers to a junction between two different materials or between materials of different compositions). These heterojunctions introduce discontinuities forming square potential wells into the band structure of the said material and in a direction substantially perpendicular to the said heterojunctions. These wells constitute reservoirs in which charge carriers of a given type are trapped. This material is placed in an electric field, called the working field, which is applied in a direction substantially perpendicular to the said heterojunctions. The multiplication of the charge carriers of the said type is set off by the injection of at least one charge carrier of the same type. This charge carrier is accelerated by the working field and thus acquires sufficient energy for it to be capable, through impact ionizing, of ejecting a charge carrier of the said type out of the potential well where it is confined. The charge carriers of the said type are guided by the working field. This ejection process is repeated from one potential well to the next and constitutes an avalanche multiplication process. Unfortunately, the crystalline growth of heterostructures of this type is extremely difficult to achieve: on the one hand, it calls for precise control over a great many parameters and, on the other hand, it is difficult to make "clean" interfaces, namely interfaces that are clearly defined in terms of crystallography.
Furthermore, the potential wells obtained in the band structure of a material of this type are shallow. There is therefore a high probability that a charge carrier will possess sufficient thermal energy to spontaneously leave the potential well where it is trapped. Consequently, this device possesses high thermal noise.
An object of the present invention is a charge carrier multiplying device (hereinafter called CCMD) based on the same working principle as the prior art device described in the previous paragraph but comprising an electrically semi-conductive material consisting of a single semiconductor of homogeneous composition in which substantially parallel layers are made, the said layers being n-doped or p-doped depending on the type of charge carriers which it is sought to multiply. For the CCMD of the present invention can multiply:
Either electrons, in which case it is an electron multiplying device (hereinafter called EMD);
Or holes, in which case, it is a hole multiplying device (hereinafter called HMD).
These layers are separated by portions of the said electrically semi-conductive material and the thicknesses of the said layers are far smaller than the dimensions of the said portions in a direction substantially perpendicular to the said layers.
The crystalline growth of a single homogeneous-composition semiconductor of this type, containing substantially parallel doped layers, is far less difficult to achieve than the growth of a heterostructure such as that of the prior art, for there is a far smaller number of parameters to be monitored.
Consequently, a single semiconductor of this type containing doped layers is far easier to manufacture on an industrial scale than a heterostructure of the type described previously.
Furthermore, the doping of a single semiconductor of this type makes it possible to obtain far deeper potential wells than the square potential wells obtained with the heterojunctions of the heterostructure described above. The CCMD of the present invention has a much lower level of thermal noise than the device of the prior art.